1. Filed of Invention
The present invention relates generally to semiconductors and more specifically to a silicon-based PT/PZT/PT sandwich structure.
2. Description of Related Art
Recently, ferroelectric thin films are widely investigated for applications in sensors, actuators, non-volatile memories and photoelectric devices. Lead Zirconate titanate (PZT) thin films are especially well known materials because of their remarkable dielectric, peizoelectric, ferroelectric and photoelectric properties. Therefore it would highly desirable to integrate ferroelectric materials in fabrication of semiconductor devices especially in fabricating memory cell capacitor where a high operating speed is highly demanded. One problem is even though such a design rule is well known, given the complexity integrating these materials and their manufacturing process, and even their use becomes less attractive when cost and size are critical.
Therefore, various deposition methods, such as sol-gel, metalorganic chemical vapor deposition (MOCVD), pulse laser deposition, and sputtering were investigated to fabricate silicon-based PZT thin films as it is highly desirable realize a method of easy operation, low cost, low annealing temperatures and compatibility with semiconductor technology. Among various fabrication techniques, sol-gel method has attracted considerable interest because it can offer several advantages such as simplicity, low cost and easy composition control. In 1993, C. K. Kwok and S. B. Desu presented a novel PZT/PT deposition method to include a PT seeding layer between PZT thin layer and a substrate which can offer nucleation sites and reduce the activation energy for crystallization of PZT thin films.
The present invention provides a silicon based PT/PZT/PT sandwich structure and a method for fabricating the same for further improving the electrical properties of the device and reducing the manufacturing cost.
The present invention provides a silicon based PT/PZT/PT sandwich structure having improved ferroelectric properties and a method for manufacturing the same.
The present invention provides a PT/PZT/PT sandwich structure that can be used for fabricating a memory cell capacitor so that the operating speed of the memory cell can be substantially enhanced.
The present invention provides a sol-gel process for depositing the ferroelectric thin films for fabricating a silicon-based PT/PZT/PT sandwich structure so that the thermal budget and cost of fabrication are both substantially reduced. The electrical properties of the ferroelectric thin films are substantially improved.
According to the preferred embodiment of the present invention, a silicon based PT/PZT/PT structure comprising a silicon semiconductor substrate. A dielectric layer is formed over a semiconductor substrate. The dielectric layer preferably comprises a silicon dioxide layer. A first and the second conductive films are sequentially formed over the dielectric layer. The first conductive film preferably comprises a Pt film, and the second conductive film comprises a Ti film. A first ferroelectric film is formed over the first and second conductive films. The first ferroelectric film preferably comprises a PT film. A second ferroelectric film is formed over the first ferroelectric film. The second ferroelectric film preferably comprises a PZT film. A third ferroelectric film is formed over the second ferroelectric film. The third ferroelectric film preferably comprises a PT film. The resulting structure is annealed. A third and fourth conductive films are sequentially formed over the third ferroelectric layer. The third and fourth conductive films preferably comprises a Pt and Ti films respectively. The third and fourth conductive films are patterned.
It is to be further understood by those skilled in the art that since the dielectric constant of PT film is smaller compared to PZT film, therefore by placing a PZT film in between two PT thin films, the electrical properties can be substantially improved. Therefore the operating speed of the device can be substantially increased.
It is to be understood by those skilled in the art that the first ferroelectric film comprises a PT film, since the PT film has a similar lattice constant and crystallographic symmetry as those of the second ferroelectric film which comprises a PZT film, therefore PZT film of high quality can be grown on the PT film. Since the nucleation barriers of PT film is lower compared to PZT film, PT film can offer excellent nucleation sites for facilitating complete crystallization of PZT film.
It is to be further understood by those skilled in the art that since the perovskite transformation is also a nucleation reaction, PT film can also effectively facilitate the pervoskite transformation at a comparatively lower temperature. A typical perovskite transformation process or an annealing process require high temperature, for example for PZT film, a temperature of about 900xc2x0 C. is required to effect the perovskite transformation. By forming a PZT film on the PT film, the perovskite transformation temperature can be effectively reduced from 900xc2x0 C. to 700xc2x0 C. Therefore, PT can effectively reduce the fabrication thermal budget thus, the fabrication cost can be substantially reduced. Since the perovskite transformation can be achieved at a lower temperature, there is no risk of adverse effect on the device due to thermal stress, therefore the reliability of the device can be increased.